Abstracts

The frugivorous diets of Blackcaps and Garden Warblers in Mediterranean shrubland, determined by faecal analysis, are described. Variation in fruit consumption is related to the fruit supply and the relation with body mass and fat accumulation is examined. The overall diet was highly similar in the two study years but the rankings of the fruit species consumed in the same month of different years differed, suggesting that birds were adjusting the diet to variations in phenology and fruiting intensity. Regression analysis showed that fruit availability was the main factor correlated with fruit consumption. Both warbler species selected fruits with high energy-value and showed negative responses to fruit size only in the year when fruit supply was exceptionally high. Both species selected a small subset of the possible combinations of fruits (meals), including a large fraction of one or two major fruit species and small amounts of several minor species or insects. Meals of several fruit species were consumed more frequently than expected under the hypothesis of random and independent assortment of component fruits. Minor fruit species were consumed more frequently than expected from their relative availability. Pulp of minor fruits was significantly higher in minerals than the pulp of majors, that characteristically showed a higher energy content. Alkaloids were more frequently present in minor species. Both the accumulation of fat and body mass gain between recaptures of ringed birds were negatively correlated with the amount of fruit in their diet. A simulation of the diet under the premise of a total reliance on fruit food showed that warblers eating only fruits should obtain significantly reduced amounts of protein and minerals but increased energy income. Inclusion of minor amounts of insect prey dramatically increased the protein yield of the meals and this may be necessary to increase body mass and fattening. The implications of these patterns of dietary variation and fruit choice on plant seed dispersal are discussed.